title: Relative contributions of norepinephrine and serotonin transporters to antinociceptive synergy between monoamine reuptake inhibitors and morphine in the rat formalin model.
abstract: Multimodal analgesia is designed to optimize pain relief by coadministering drugs with distinct mechanisms of action or by combining multiple pharmacologies within a single molecule. In clinical settings, combinations of monoamine reuptake inhibitors and opioid receptor agonists have been explored and one currently available analgesic, tapentadol, functions as both a µ-opioid receptor agonist and a norepinephrine transporter inhibitor. However, it is unclear whether the combination of selective norepinephrine reuptake inhibition and µ-receptor agonism achieves an optimal antinociceptive synergy. In this study, we assessed the pharmacodynamic interactions between morphine and monoamine reuptake inhibitors that possess different affinities and selectivities for norepinephrine and serotonin transporters. Using the rat formalin model, in conjunction with measurements of ex vivo transporter occupancy, we show that neither the norepinephrine-selective inhibitor, esreboxetine, nor the serotonin-selective reuptake inhibitor, fluoxetine, produce antinociceptive synergy with morphine. Atomoxetine, a monoamine reuptake inhibitor that achieves higher levels of norepinephrine than serotonin transporter occupancy, exhibited robust antinociceptive synergy with morphine. Similarly, a fixed-dose combination of esreboxetine and fluoxetine which achieves comparable levels of transporter occupancy potentiated the antinociceptive response to morphine. By contrast, duloxetine, a monoamine reuptake inhibitor that achieves higher serotonin than norepinephrine transporter occupancy, failed to potentiate the antinociceptive response to morphine. However, when duloxetine was coadministered with the 5-HT3 receptor antagonist, ondansetron, potentiation of the antinociceptive response to morphine was revealed. These results support the notion that inhibition of both serotonin and norepinephrine transporters is required for monoamine reuptake inhibitor and opioid-mediated antinociceptive synergy; yet, excess serotonin, acting via 5-HT3 receptors, may reduce the potential for synergistic interactions. Thus, in the rat formalin model, the balance between norepinephrine and serotonin transporter inhibition influences the degree of antinociceptive synergy observed between monoamine reuptake inhibitors and morphine.
authors.affiliation: Departments of Pharmacology, Theravance Inc., South San Francisco, California, United States of America.
authors.forename: Fei
authors.initials: F
authors.lastname: Shen
authors.forename: Pamela R
authors.initials: PR
authors.lastname: Tsuruda
authors.forename: Jacqueline A M
authors.initials: JA
authors.lastname: Smith
authors.forename: Glenmar P
authors.initials: GP
authors.lastname: Obedencio
authors.forename: William J
authors.initials: WJ
authors.lastname: Martin
mesh_terms: D000698:Analgesia / Q000379:methods*; D000818:Animals; D000069445:Atomoxetine Hydrochloride; D015306:Biogenic Monoamines / Q000378:metabolism; D002853:Chromatography, Liquid; D004357:Drug Synergism; D000068736:Duloxetine Hydrochloride; D005473:Fluoxetine; D005557:Formaldehyde; D009020:Morphine / Q000378:metabolism*; D009025:Morpholines; D014179:Neurotransmitter Uptake Inhibitors / Q000378:metabolism* / Q000493:pharmacokinetics; D059226:Nociceptive Pain / Q000188:drug therapy*; D050484:Norepinephrine Plasma Membrane Transport Proteins / Q000378:metabolism*; D017294:Ondansetron; D011437:Propylamines; D051381:Rats; D045442:Rotarod Performance Test; D050486:Serotonin Plasma Membrane Transport Proteins / Q000378:metabolism*; D053719:Tandem Mass Spectrometry; D013876:Thiophenes
chemical_list: D015306:Biogenic Monoamines; D009025:Morpholines; D014179:Neurotransmitter Uptake Inhibitors; D050484:Norepinephrine Plasma Membrane Transport Proteins; D011437:Propylamines; D050486:Serotonin Plasma Membrane Transport Proteins; D013876:Thiophenes; D005473:Fluoxetine; D005557:Formaldehyde; D017294:Ondansetron; D000069445:Atomoxetine Hydrochloride; D009020:Morphine; D000068736:Duloxetine Hydrochloride; C541385:esreboxetine